In "Plain Sex," Alan H. Goldman makes two main arguments. The first is that the value judgments people make about sex are generally based on premises that cannot be defended, either on deontological or consequentialist grounds. The second is that being a moral objectivist does not necessarily entail being a conservative. Both deontological and consequentialist ethics can allow liberalism in the sexual sphere.
Goldman first points out that people who moralize about sex tend to make unsupported assertions about its purpose. These range from the idea that sex is purely a means of reproduction to the idea that sex must always be an expression of love. Goldman does not believe that there is any reason to assert that sex has to be either of these things. However, he notes that if you do believe this, then you will be likely to condemn any sexual act that does not fulfill the purpose you have assigned to it as immoral. If you want to condemn homosexuality, you can say that it is immoral because no children can result, but you have no reason to say that this is the only criterion for sex to be moral, unless you resort to the naturalistic fallacy. Similarly, you might condemn two people who have sex for physical pleasure because they do not love each other, but you have no basis for the assertion that it is immoral to have sex with someone you do not love.
Goldman also argues that both utilitarianism and Kantian ethics can support sexual liberalism, as long as no one is harmed by this. If two adults consent to sexual activity, and enjoy it, then it is not immoral by the standards of either philosophy. In both his points, Goldman's argument is well-supported, valid, and sound, since he develops his points logically, without fallacies or flaws.